Our current mixed chimerism has reliably achieved long-term kidney graft survival exclusively in monkeys (60%) displaying low memory T cell reactivity against their donors prior to transplantation in cynomolgus monkeys (5-7). The presence of regulatory T cells (Tregs) capable of suppressing anti-donor inflammatory responses in vitro was consistently detected in tolerant animals. On the other hand, monkeys displaying high anti-donor memory responsiveness rejected kidney allografts in an acute fashion (6,7). Finally, it is important to note that approximately half of the long-term surviving kidney allografts succumb eventually to chronic rejection (8). These observations stress the need to improve our protocol in order to achieve tolerance to kidney and islet/kidney transplants in 100% of monkeys. Projects 1 and 2 of this program project propose a series of novel strategies designed to induce tolerance to kidney and islet/kidney allografts in monkeys. Successful tolerance induction in both of these studies rely on the basic principles that prevention/suppression of pro-inflammatory alloimmunity combined with enhancement of immune regulation will promote tolerogenesis. Project 3 is 1) to investigate the mechanisms by which the in vivo treatments described in Projects 1 and 2 influence the immune response in monkeys transplanted with allogeneic kidneys, islets or islet/kidneys and, 2) elucidate the nature of the cells, soluble factors and immunological mechanisms involved in induction and maintenance of transplant tolerance. This knowledge will help us determine how to select the appropriate donor/recipient combinations and to refine the treatments proposed in Project 1 and 2 in order to reliably achieve tolerance to kidney and islet transplants in primates. To study this, we propose the following Aims: Specific aim 1. Investigate donor hematopoietic mixed chimerism, leukocyte recovery and deleterious alloimmune T cell responses in transplanted monkeys Specific aim 2. Investigate regulatory T cell responses associated with transplant tolerance Specific aim 3. Investigate B cell responses and their contribution to rejection and tolerance Understanding the mechanisms by which tolerance is induced and maintained in our primate transplant model should significantly expand the successful application of tolerance protocols in clinical transplantation and for the treatment of immune-mediated diseases requiring tolerogenesis of harmful memory T cells in patients such as autoimmune disorders and allergies.